The tendency in telecommunication systems is increasingly to an environment where an application system (known also as an application network) is built onto or connected to an existing telecommunication system (known as a bearer system or bearer network) and uses the existing telecommunication system as a service bearer between a service provider and a user. For example in what are known as third generation mobile systems, such as the UMTS (Universal Mobile Communications system), the existing telecommunication system provides a user with a wireless connection to external systems, such as the IMS (IP multimedia subsystem) and their services, such as IP telephony (IPT). IP telephony is a general term covering services from standard voice over IP (VoIP) to multimedia applications that use IP data, voice and video in IP telephony. In addition to IP telephony, said telecommunication system supports other applications, such as access to the Internet or an intranet. An IP call, in turn, refers to a call that uses an IP-based user data stream and signalling. User data may contain various components, such as voice, video image and data. In addition to calls, IP telephony may comprise call-like services that may be unidirectional, directed to a group (or groups) or broadcasts within a given area, for example.
A mobile system service gaining popularity is a prepaid subscription. In a prepaid subscription, calls are not charged for retroactively, but the account of the prepaid subscription is billed online during calls. A prepaid subscription can usually be used for chargeable calls as long as credits exit on the subscription account, i.e. the entire prepaid sum has not been used.
A prepaid subscription will also be offered to users of external systems in such a way that a user needs only one subscription, which both the bearer system and the application system use for charging. For example in online charging in the IP multimedia subsystem (IMS), both the bearer system and the IMS deliver their charging records to a charging system that attends to the online charging in the IMS and comprises a correlation function enabling the matching of the charging records delivered by the bearer system and the IMS to the right subscription and the same service.
The problem in the above arrangement is that when a connection is being set up, the bearer system does not know if a bearer service to be charged via the IMS is involved or a service that the bearer system has to charge for itself. To ensure that a subscription is charged for the use of a bearer system, the bearer network node collecting charging data should be given instructions to transmit charging records to the charging system of the bearer system. To ensure that a subscription is charged for the IMS service, the IMS node collecting charging data should be given instructions to transmit charging records to the charging system of the IMS. In that case there is a risk that the usage of the bearer system is charged twice, the other charging being included in the service charge of the IMS. To avoid double charging, all charging records should be gone through and overlapping charging records should be identified. This is slow and is not suited to online charging.
The same problem exists also in other applications of online charging, such as a postpaid subscription on credit control. It is a subscription that is charged afterwards and given a maximum amount to be billed, i.e. the maximum amount of the bill within a charging period.